1. Field of the Invention
The present invention relates generally to the field of radars and more particularly to the field of CW (continuous wave) radars configured for target range detection.
2. Background Discussion
Radars are generally known as active electronic apparatus which detect objects by radiating microwave energy, in the form of electromagnetic waves, and by processing return signals from reflecting objects. A widely held conception is that radars transmit short bursts of energy and measure the round trip time of the pulses to a target object and back to a receiver, the target range being computed from the round trip pulse time and the known velovity of wave propagation.
Typically pulsed radars generate a train of short pulses, the length of each pulse in the train typically being only a few microseconds and the pulse repetition frequency (PRF) being typically several hundred pulses per second. Each pulse consists of a packet or burst of an RF wave having a typical frequency in the high megahertz to gigahertz range. Target velocity and/or closing rate may also be determined by pulsed radars by examining the doppler frequency shift in the reflected wave caused by target movement, such radars being called pulsed doppler radars.
As an alternative to pulsed doppler radars for some applications, particularly for ground based low altitude surveillance in a military environment, narrow bandwith CW (continuous wave) radars may, because of advantages in target visibility, be used. Unlike pulsed radars, CW radars, in their simplest form, transmit a single sinusoidal wave, the time-delayed, received signals being mixed with the transmitted carrier frequency.
However, with advances in digital signal processing techniques, weapons requirements and Low Probability of Interference (LPI) radar technologies, wide bandwidth (high resolution), frequency agile, track-while-scan ground surveillance radars must be developed. Current CW radars use RF/IF cancellation techniques to reject transmitter-receiver signal leakage and short range clutter return signals. Although CW radars using pure CW waveforms exhibit excellent leakage/clutter immunity, such radars cannot measure target range. To provide target range measurement capability, some form of phase or frequency modulation must be imparted to the transmit waveform. In general, the range resolution improves with the bandwidth of such modulations, while the susceptibilty of the radar to leakage/clutter increases with modulation bandwidth. Conventional leakage/clutter cancellation or rejection techniques are not applicable to wide bandwidth signals. Improvements are, therefore, needed in wide bandwidth techniques that are compatible with high performance CW surveillance radars.